Conventional thermoformable thermoplastic resin sheets typically are made from resins such as polyvinyl chloride, polystyrene-based resins and the like. Among these thermoplastic resins, however, polyvinyl chlorides have disadvantages in respect to hygiene, heat resistance, moisture proofness and other properties. Moreover, incineration of these materials causes emission of chlorine-containing gases. Polystyrene-based resins also show deficiencies in respect to heat resistance, impact strength, moisture-proofness and other properties. Notwithstanding these disadvantages and deficiencies, sheets of these thermoplastic resins are widely used as thermoformed packaging material in many fields.
Conventional thermoforming processes generally involve heating a thermoplastic sheet above its softening point, forming the softened sheet and allowing the formed sheet to cool and harden. Polypropylene, being a highly crystalline polymer, must be heated up to its melting temperature (Tm) of about 160.degree. C. in order to be thermoformed by conventional thermoforming processes. The flexural modulus of polypropylene can decrease by more than two orders of magnitude at it nears its melting temperature which can thereby cause sheet made of polypropylene to undergo excessive sag during thermoforming. Also, polypropylene does not exhibit the rubbery plateau that is characteristic of glassy polymers when such polymers are heated above their glass transition temperature (Tg). Nevertheless, resinous polymers of propylene have been used increasingly in recent years in place of polyvinyl chloride and polystyrene-based resins by virtue of their excellent strength, rigidity, heat resistance, moisture proofness and other desirable properties.
The market for thermoformed plastic products has undergone rapid growth in recent years, and polypropylene-based resins have the potential to become a premium material for this market. That such resins are difficult to thermoform, as described above, has limited their use in this high-growth area. Various methods have been attempted to lessen these thermoforming difficulties.
One such method has been developed by Shell Development Co. wherein a solid phase pressure forming process, known an SPPF, is utilized. Although the SPPF process allows a hot, but not molten, sheet to be thermoformed just below its crystalline melting point, other limiting conditions are introduced such as the need for specialized, expensive thermoforming equipment, restricted depth of draw, limited draw ratio, and high levels of built-in stress.
Still other methods of attempting to overcome the thermoforming difficulties have taken the approach of tailoring the molecular weight of the polypropylene resin, thereby making it possible to process extruded sheets on conventional thermoforming equipment. In order to reduce the degree of sagging of the heated sheet as it nears its crystallization temperature, the melt flow rate of the polypropylene resin has had to be lowered to fractional values, typically less than 0.25 dg/min. The resulting high melt viscosity creates another problem; namely, that of producing sheet at economical extrusion production rates. Attempts have been made to overcome this problem by broadening the molecular weight distribution of the polypropylene to improve extrudability.
Thermoformed articles made by the processes described above depending on the products contained in them and conditions under which the articles are microwaved, can undergo change in dimensional integrity.
U.S. Pat. No. 4,680,157 discloses a method for preparing a sheet of polypropylene having excellent transparency and surface properties as well as thermoformability including a slight stretching of the sheet and optionally alpha-spherulite nucleating agents with articles vacuum thermoformed from sheet heated at 153.degree. to 158.degree. C.
U.S. Pat. No. 4,567,089 discloses a propylene polymer laminated sheet for surface gloss, appearance and impact resistance with a surface layer comprising crystalline polypropylene and up to 5 wt % of an inorganic or organic alpha-spherulite nucleating agent and a second layer comprising polypropylene, an ethylene polymer and an inorganic filler.
Beta-spherulite nucleating agents useful in compositions for forming unstretched films up to 0.4 mm in thickness which can be made porous by extraction of beta-spherulites, stretching the film or a combination of extraction and stretching are disclosed for the production of porous films and processes for making such films in U.S. Pat. Nos. 4,386,129 and 4,975,469, U.S. patent application (abandoned) Ser. No. 07/633,087, filed Dec. 21, 1990, in the name of P. Jacoby, et al., and U.S. Pat. No. 5,176,953 in the name of P. Jacoby, et al., all commonly assigned to the present assignee.
In spite of the showing of the use of beta-spherulite nucleating agents in the formation of microporous films and the various techniques for thermoforming sheets of polypropylene-based resins including the use of alpha-spherulite nucleating agents, there remains a need for resinous polymers of propylene which can be formed into sheet under facile conditions and competitive production rates which can be used for thermoforming articles. Such sheet would desirably also be thermoformable on conventional thermoforming equipment at increased production rates with the resultant thermoformed articles having desirable improved end-use properties such as microwaveability and low-temperature impact resistance.
Applicants have unexpectedly found that polymeric compositions comprising a resinous polymer of propylene and an effective amount of a beta-spherulite nucleating agent are useful for preparing a thermoformable sheet, particularly polypropylene-based compositions having sufficient melt flow rate for the facile and efficient production of sheet, and the thermoforming of articles from such sheet on conventional thermoforming equipment.
It is an object of this invention to provide an improved thermoformable sheet.
Another object of this invention is to provide an improved thermoformable sheet comprising a resinous polymer of propylene and an effective amount of beta-spherulites.
A further object of this invention is to provide a method for thermoforming a sheet comprising a resinous polymer of propylene and an effective amount of beta-spherulites.
A still further object of this invention is to provide articles thermoformed from such improved thermoformable beta-spherulite nucleated resinous polymers of propylene.
Advantageously, the thermoformable sheet of this invention comprises one or more layers of a crystalline resinous polymer of propylene having beta-spherulites present at a K-value of about 0.3 to 0.95 which can be thermoformed at significantly higher production rates and the sheet produces thermoformed articles which exhibit improved sidewall strength, reduced warp, and improved microwaveability compared to articles produced from conventional alpha-spherulite nucleated or non-nucleated polypropylene-based resins. Additional advantages are found in embodiments of the invention comprising multilayer sheets which include interior layers of beta-spherulite nucleated resinous polymer of propylene and exterior layers of polypropylene-based resins such as ethylene-propylene impact copolymer for improved low-temperature impact resistance.